This invention relates to systems for continuous monitoring of a condition of a material such as material level and, in particular, to systems developing an output varying in relation to changing admittance.
U.S. Pat. No. 4,146,834, assigned to the assignee of this invention, describes various admittance responsive measuring systems for sensing the condition of materials. More particularly, two-wire transmitter systems, each working with an admittance probe and each including an admittance responsive network generating a signal representing the condition of the monitored material, are described. Each of the various described systems couples an admittance probe in a balanced bridge circuit and drives the probe and bridge circuit by means of an oscillator having a fixed frequency output. Such bridge systems provide very accurate measurement of admittance, particularly at and around the reference admittance point. However, this accuracy degenerates as the measured admittance increasingly differs from the reference admittance. Moreover, such systems typically employ transformers, phase detectors, demodulators, or and other complex, expensive, circuitry.
A much simpler two-wire capacitive, material level measuring system is described in U.S. Pat. 4,347,740. There, a capacitive measuring probe is connected to one input of an operational amplifier and is connected to the output of the operational amplifier through a resistor to form a free-running oscillator. The frequency of the oscillator output is, generally speaking, inversely related to the magnitude of the capacitance being developed by the probe and the material. A switching transistor is connected to the output of operational amplifier to generate a changing current output signal. The operational amplifier output is also passed back to the probe through either of two resistors, the values of which are selected to change the period of the oscillating signal developed in the free running oscillator, to drive the probe.
The system of U.S. Pat. 4,347,740 is much simpler and less expensive than the systems described in U.S. Pat. No. 4,146,834. However, the former invention has its own set of limitations. First, a resistance between the probe electrode and ground, which would be present with any material which is not perfectly insulative, produces a current which will affect the charge and discharge times of the probe capacitance, thereby changing the period and thus the frequency of the free-running oscillator. Asymmetric charge/discharge impedences of the system tend to aggravate the errors caused by the resistance.
U.S. Pat. No. 4,515,015 describes a capacitive level sensing device improved over that described in U.S. Pat. No. 4,347,740. This system uses an insulated probe electrode and a charging resistor having a resistance which is small with respect to the resistance of the material being measured so that a buildup or coating of the material has a diminished effect on the measurements. This does not prevent a stray current from being generated with the probe which would affect the frequency of the output signal of the oscillator.